In the lamination of packaging materials, for example for liquid packages, it is normal practice to start from a web of paperboard or cardboard, one or both sides of the web being coated with different types of films or material layers in order for the finished packaging material to attain the desired properties. Those layers which are employed for coating the paperboard or cardboard material are principally different types of plastic films, but different types of metal foils (e.g. Alifoil) may be employed. The plastic layers and the possible metal foil fulfil the purpose of preventing action between the product which is to be packed and the packaging material. Another purpose is to prevent e.g. oxygen from penetrating into the package.
Most generally, the web of paperboard or cardboard is delivered in the form of a magazine reel. The magazine reel is applied in one end of a lamination machine, which comprises a number of rollers which together form a path for the web. In order to realise the compressive force necessary for the lamination, generally two rollers are applied close to one another, so that a nip is formed between the rollers. In order to obtain a satisfactory adhesion between, for example, a plastic film and the paperboard/cardboard, the pressure in the nip should be maintained for a certain time at a given pressure. In general, it may be said that if the pressure is low, the time should be long, and vice versa.
One problem occurs if the web of paperboard or cardboard is of low density, i.e. is “fluffy”; if the intention is to maintain the low density also after the nip between the rollers, the pressure should be low (i.e. the clearance between the rollers should be large), otherwise the web will be compressed by the pressure in the nip. As was mentioned previously, the adhesion between the web and the film which is to be laminated depends upon the time during which the web is exposed to pressure as well as the pressure at which the web is exposed. If the pressure is low, the time spent in the nip should thus be long in order to reach the same level of adhesion.
There are two ways of extending the time during which a nip between two rollers subjects a web to pressure: the first method is to reduce the web speed, but from the viewpoint of production economics this is less suitable. The other possibility is to increase the diameter of the rollers, but the point is soon reached where the size of the rollers becomes unreasonable.
Moreover, the prior art lamination machines are designed for a given lamination pressure. This implies that the rollers may have a slightly convex form, which compensates for the outward flexing of the rollers, and at a certain compressive force between the rollers gives a uniform pressure throughout the entire width of the rollers. If the force between the rollers is increased or reduced, the convexity of the rollers will not correspond to the outward flexing of the rollers, for which reason the range within which it is possible to modify the compressive force between the rollers is limited.
EP 1 345 756 describes a roller comprising an inner, rigid core surrounded by two layers of resilient material. If one such roller is employed there will, granted, be a slightly longer nip, but the major reason for using such a roller is that there will be obtained a nip which is relatively insensitive to variations in the thickness of the web.